This invention relates to emulsion polymers, which provide flame retardancy, and processes for making such polymers. The emulsion polymerization processes employed forms non-homogeneous particles, such as core-shell particles. More particularly, the invention is directed to a latex where the particles have a relatively hard, vinyl chloride containing core and a relatively soft acrylic shell. The latexes are useful in coatings, as well as textile and nonwoven applications. They provide, in addition to flame retardancy, improved film forming properties, chemical resistance, abrasion resistance, and heat and/or dielectric sealability.
Currently copolymers based on ethylene and vinyl chloride are used as flame-retardant binders for woven and nonwoven fabrics. These copolymers are considered to exhibit a good overall balance of required properties. However, one of the drawbacks of this technology is that production of polymeric products based upon ethylene-vinyl chloride copolymers requires relatively expensive high pressure equipment due to the high pressure of ethylene during polymerization, and so these polymers tend to be expensive to manufacture.
Acrylate/vinyl chloride copolymers would be expected to be less expensive to manufacture, since they can be made in less expensive, lower pressure rated equipment. But, in order to deliver flame retarding properties, compositions based upon such copolymers require a relatively high chlorine content. Generally, the higher the chlorine content, the higher the minimum film-forming temperature (MFFT) and the harder or stiffer the hand. So, generally speaking, compositions based upon acrylate-vinyl chloride copolymers, where there is a high chlorine content, generally exhibit relatively high and undesirable, MFFT's. The MFFT is the temperature point at which the latex will form a continuous film. It is often desirable that the latex function, for example, as a binder at room temperature or below. Therefore, high MFFT's mean that the utility of the latex is limited.
One approach to alter the properties of emulsion polymers utilized in multiple-phase latex systems to make low MFFT latexes based on hard monomers, i.e., monomers exhibiting a relatively high polymer glass transition temperature (Tg), has been to form a soft shell (usually acrylic based) around a hard core (such as formed from styrenic or methacrylic monomers). Poly(vinyl chloride) and vinyl chloride copolymers are of particular interest as core materials due to their flame retardancy; oil, chemical, and water resistance; strength; abrasion resistance; compoundability; and low cost. However, polymers made of acrylic and methacrylic monomers, particularly those with an alkyl side chain of up to six carbon atoms, ie., methyl through hexyl (meth)acrylates, including commercially used ethyl and n-butyl acrylates, are miscible with PVC. See D. J. Walsh et al., Polymer, 25, 495 (1984); 23, 1965 (1982); and 21, 1330 (1980).
To create core-shell structures, the multiple polymer phases must phase separate by virtue of their immiscibility, by their lack of interdiffusion, or by some other process. If phase separation does not occur or is not maintained in the finished emulsion polymeric particles, then the behavior of the polymeric particle may approach that of a particle formed from a homogeneous alloy of polymers. The result is generally inferior properties as compared to phase separated particles in which the particle morphology, or the location of the phases within the particles, can be controlled. It is understood that the level of phase separation of polymer phases in core-shell particles is never complete. Some degree of miscibility will be apparent at the interphase, and core-shell particles may be produced with significant levels of miscibility between the phases. The extent of phase separation, however, is a critical parameter in controlling the final morphology of the polymer particles, and thus the performance of the final emulsion produced. Since many common acrylate polymers are miscible with PVC, they tend not to phase separate from PVC. Thus, the particle morphologies created are often undesirable.
European Patent Application No. 0 810 240 A1 (assigned to Elf Atochem) teaches a latex in the form of particles having a vinyl chloride core and an external acrylate copolymer layer, which is for use in paints and plastisols. The external acrylate copolymer is formed from the polymerization of monomer(s) which could be alkyl methacrylates or acrylates, where the alkyl group is between 1 and 8 carbon atoms, and/or vinyl esters of mono and polycarboxylic acids, and in the presence of a vinyl chloride seed polymer via a suspension polymerization process. Although the application discloses that the weight ratio of vinyl chloride seed polymer to external monomers can be between 0.02 and 10, in fact, in all the examples, the ratios employed were 50% by weight or less. Therefore, the compositions taught by the Elf Atochem patent application would be considered not to be suitable for providing flame retardancy.
The core-shell composition of the present invention overcomes the inherent obstacles of both the ethylene--vinyl chloride copolymers and the PVC core--acrylic shell polymers such as described in European Patent Application No. 0 810 240 A1, and provides a low MFFT in conjunction with high chlorine content, thereby yielding excellent flame retardancy. Other benefits over existing technologies will become clear from the following description.